The present invention relates to the manufacture of optical fibers.
Optical fiber used in communication systems typically includes a core of glass surrounded by a cladding also formed from glass having different optical properties from the core. The fiber typically is covered with a protective outer coating. Such fibers can be made by drawing a thin strand from a heated, partially molten preform formed from glass having the correct composition to make the core surrounded by a layer of glass having the appropriate composition to make the cladding. As a strand of soft, molten glass is pulled from the preform, both the core glass and the cladding glass stretch. The core remains in the middle and the cladding remains on the outside, thus forming the composite core and cladding structure of the finished fiber. As the fiber is pulled away from the preform, it cools and solidifies, and the coating is applied. These processes are performed at high speeds so that the fiber is drawn at high rates.
In operation of an optical communication system, light applied at one end of the fiber is pulsed or progressively varied in accordance with the information to be transmitted. The pulses or progressively varying light are received at the other end of the fiber. The speed at which light passes along a fiber depends upon many factors including the optical properties of the materials making up the core and cladding and, the diameter of the core. The fibers commonly used for optical data transmission systems are so-called "single mode" fibers. In these fibers, the core diameter is small enough that all of the light must pass through the core in a so-called "fundamental" or "HE11" mode of transmission. Full discussion of transmission modes in optical fibers is beyond the scope of this disclosure. However, the fundamental or HE11 mode can be regarded as propagation of light straight along the axis of the core, as opposed to higher-ordered modes which can be thought of as propagation of light in a zig-zag pattern.
In a theoretically perfect single mode fiber, because all of the light passes through the fiber in the same mode, all light of a given wavelength will pass along the length of the fiber with the same velocity. However, the light passing along the fiber typically includes portions having different polarizations, i.e., different orientation of the electromagnetic waves constituting the light. If the fiber core is not perfectly cylindrical, but instead is out of round so that it has long and short diameters, light of one polarization will have its electrical waves aligned with a long diameter of the core whereas light of the other polarization will have its electrical waves aligned with the short diameter of the core. In this case, the effective diameter of the fiber core will be different for light of one polarization than for light of another polarization. Portions of light having different polarizations will travel at different velocities. Stated another way, the fiber has a "slow" axis in one direction perpendicular to its length, and a "fast" axis in the other direction perpendicular to its length.
Light having a direction of polarization aligned with the fast axis travels more rapidly than light having a direction of polarization aligned with the slow axis. As a result, the two polarization modes propagate with different propagation constants (.beta..sub.1 and .beta..sub.2). The difference between the propagation constants is termed birefringence (.DELTA..beta.), the magnitude of the birefringence being given by the difference in the propagation constants of the two orthogonal modes: EQU .DELTA..beta.=.beta..sub.1 -.beta..sub.2.
Birefringence causes the polarization state of light propagating in the fiber to evolve periodically along the length of the fiber. The distance required for the polarization to return to its original state is the fiber beat length (L.sub.b), which is inversely proportional to the fiber birefringence. In particular, the beat length Lb is given by: EQU L.sub.b =2.eta./.DELTA..beta.
Accordingly, fibers with more birefringence have shorter beat lengths and vice versa. Typical beat lengths observed in practice range from as short as 2-3 millimeters (a high birefringence fiber) to as long as 10-50 meters (a low birefringence fiber).
In addition to causing periodic changes in the polarization state of light traveling in a fiber, the presence of birefringence means that the two polarization modes travel at different group velocities, the difference increasing as the birefringence increases. The differential time delay between the two polarization modes is called polarization mode dispersion, or PMD. PMD causes signal distortion which is very harmful for high bit rate systems and analog communication systems.
This phenomenon is referred to in the art of fiber optic communication as polarization mode dispersion or "PMD". Imperfections in the fiber other than differences in core diameter can also contribute to PMD. PMD causes distortion of the light pulses or waves transmitted along the fiber, thus reducing the signal quality and limiting the rate at which information can be passed along the fiber.
One method of reducing the effects of PMD is to continually reorient the fast and slow axis of the fiber. This can be accomplished by spinning the fiber as it is drawn, so that the slow axis and the fast axis of the fiber are repeatedly interchanged along the length of the fiber. Thus, at one point along the length of the fiber the slow axis points in a first direction perpendicular to the length of the fiber and the fast axis points in a second direction perpendicular to the length of the fiber and perpendicular to the first direction. At another point along the length of the fiber, the fast axis points in the first direction and the slow axis points in the second direction. In a fiber with spin, the fast axis traces a generally helical path. The magnitude of the spin can be expressed as the number of turns per unit length of such helix, i.e., the number of times per unit length of fiber that the directions of the fast and slow axes interchange. The direction of the spin corresponds to the direction of the helix traced by the fast axis, either right-handed or left-handed. In a fiber with the appropriate spin, the effects caused by the fast and stow axes are substantially eliminated and all light travels with the same velocity. To provide optimum PMD suppression, it is normally desirable to vary the magnitude and direction of the spin along the length of the fiber.
Various attempts have been made to impart spin to the optical fiber during the production process discussed above. For example, as disclosed in Rashleigh, Navy Technical Disclosure Bulletin, Volume 5, Number 12, December 1980, Navy Tech. Cat. No. 4906, a twisted fiber can be prepared by rotating the preform about its axis while drawing the fiber from the preform. A similar approach, more generally stated as "continuous relative rotation between the preform and the drawn fiber" is disclosed in International Patent Publication WO 83/00232. As disclosed, for example, in U.S. Pat. No. 4,509,968, the process involving rotation of the preform leads to considerable practical disadvantages. The preform is a massive, soft object which must be maintained at a high temperature. The '968 patent, therefore, proposes to produce a helical or "chrialic" structure in the fiber by feeding the fiber through a set of nips at the cold or downstream end of the fiber drawing process while continually spinning the frame holding the nips. A complex arrangement of a frame and fiber takeup drum is used in this process to transfer the fiber from the spinning nips to the takeup drum.
Hart, Jr., et al. U.S. Pat. Nos. 5,418,881 and 5,298,047 disclose another process for making fibers with spin of alternating clockwise and counterclockwise directions. In this process, the cold end of the fiber passes around a roller while the roller rotates about an axis perpendicular to the longitudinal or upstream-to-downstream direction of the fiber. The roller is periodically moved so that the fiber tends to roll along the surface of the roller, parallel to the axis of rotation of the roller. The fiber periodically slips or jumps along the surface of the roller. Despite these and other efforts in the art, there are still needs for further improvements in processes for imparting a controlled spin to an optical fiber. In particular, there are needs for processes which can provide non-uniform spin, and particularly alternating spins in opposite directions to the fibers in a repeatable, controllable manner. There are corresponding needs for reliable, repeatable apparatus for imparting controlled spins to fibers. In particular, there are needs for methods and apparatus which can impart appreciable spin to a fiber in a repeatable manner during high speed fiber drawing, and which can be used in combination with conventional fiber drawing equipment and processes.